Heretofore, there have been known two methods for thermal transfer recording for the preparation of a multicolor image which utilize a thermal head printer, that is, a sublimation dye transfer recording and a fused ink transfer recording.
The sublimation dye transfer recording method comprises the steps of superposing on an image receiving sheet a transfer sheet which is composed of a support and a transfer layer comprising a sublimation ink and a binder and imagewise heating the support of the transfer sheet to sublimate the sublimation ink to form an image on the image receiving sheet. A multicolor image can be prepared using a number of color transfer sheets such as a yellow transfer sheet, a magenta transfer sheet, and a cyan transfer sheet.
The sublimation dye transfer recording, however, has the following drawbacks:
1) The gradation of image is mainly formed of variation of the sublimated dye concentration, which is varied by controlling the amount of sublimation of the dye. Such gradation is appropriate for the preparation of a photographic image, but is inappropriate for the preparation of a color proof which is utilized in the field of printing and whose gradation is formed of dots, lines, or the like, that is, area gradation. PA1 2) The image formed of sublimated dye has poor edge sharpness, and a fine line shows thinner density on its solid portion than a thick line. Such tendency causes serious problem in the quality of character image. PA1 3) The image of sublimated dye is poor in endurance. Such image cannot be used in the fields which require multicolor images resistant to heat and light. PA1 4) The sublimation dye transfer recording shows sensitivity lower than the fused ink transfer recording. Such low sensitive recording method is not preferably employable in a high speed recording method utilizing a high resolution thermal head, of which development is expected in the future. PA1 5) The recording material for the sublimation dye transfer recording is expensive, as compared with the recording material for the fused ink transfer recording. PA1 (1) Each color image (i.e., cyan image, magenta image or yellow image) of the multicolor image for color proofing should has a reflection density of at least 1.0, preferably not less than 1.2, and especially not less than 1.4, and a black image preferably has a reflection density of not less than 1.5. Thus, it is desired that the heat sensitive ink sheet has the above reflection densities. PA1 (2) An image which is produced by area gradation is satisfactory. PA1 (3) An image can be produced in the form of dot, and the formed line or point has high sharpness in the edge. PA1 (4) An ink layer (image) transferred has high transparency. PA1 (5) An ink layer has high sensitivity. PA1 (6) An image transferred onto a white paper sheet (e.g., coated paper) is extremely near to a printed image in tone and surface gloss. PA1 wherein the second image receiving layer comprises butyral resin and polymer having at least one of recurring units represented by the following formula (1): ##STR2## wherein R.sup.1 represents a hydrogen atom or a methyl group; and PA1 1) The image receiving sheet wherein the first image receiving layer contains plasticizer having molecular weight of not less than 1,000 and has Young's modulus of 10 to 10,000 kg.multidot.f/cm.sup.2 at room temperature. PA1 The image receiving sheet can be employed in combination with the heat sensitive ink sheet which comprises a support sheet and a heat sensitive ink layer having a thickness of 0.2 to 1.0 .mu.m which is formed of a heat sensitive ink material comprising 30 to 70 weight % of colored pigment, 25 to 60 weight % of amorphous organic polymer having a softening point of 40.degree. to 150.degree. C. (preferably and 0.1 to 20 weight % of nitrogen-containing compound). PA1 superposing the heat sensitive ink sheet comprising a support sheet and a heat sensitive ink layer thereon formed of heat sensitive ink material on the second image receiving layer of the image receiving sheet described above; PA1 placing imagewise a thermal head on the support of the heat sensitive ink sheet to form an image of the ink material with area gradation on the image receiving sheet; PA1 separating the support of the heat sensitive ink sheet from the image receiving sheet so that the image of the ink material is retained on the second image receiving layer; PA1 superposing the image receiving sheet on a white paper sheet in such a manner that the image of the ink material is in contact with a surface of the white paper sheet; and PA1 separating the image receiving sheet from the white paper sheet, keeping the image of the ink material having the second image receiving layer on the white paper sheet, said image of the ink material on the white paper sheet having an optical reflection density of at least 1.0. PA1 superposing the heat sensitive ink sheet comprising a support sheet and a heat sensitive ink layer thereon formed of heat sensitive ink material on the second image receiving layer of the image receiving sheet described above; PA1 irradiating a laser beam modulated by digital signals on the heat sensitive ink layer through the support of the heat sensitive ink sheet to form an image of the ink material on the image receiving sheet; PA1 separating the support of the heat sensitive ink sheet from the image receiving sheet so that the image of the heat sensitive ink material is retained on the second image receiving layer; PA1 superposing the image receiving sheet on a white paper sheet in such a manner that the image of the ink material is in contact with a surface of the white paper sheet; and PA1 separating the image receiving sheet from the white paper sheet, keeping the image of the ink material having the second image receiving layer onto the white paper sheet, said image of the ink material on the white paper sheet having an optical reflection density of at least 1.0. PA1 superposing a first heat sensitive ink sheet (such as a cyan ink sheet) ion an image receiving sheet; PA1 placing imagewise a thermal head on the support of the first heat sensitive ink sheet to form and transfer a color image (cyan image) of the heat sensitive ink material onto the image receiving sheet; PA1 separating the support of the ink sheet from the image receiving sheet so that the color image (cyan image) of the heat sensitive ink material is retained on the second image receiving layer; PA1 superposing a second heat sensitive ink sheet (such as a magenta ink sheet) on the image receiving sheet having the cyan image thereon; PA1 placing imagewise a thermal head on the support of the second heat sensitive ink sheet to form and transfer a color image (magenta image) of the heat sensitive ink material onto the image receiving sheet; PA1 separating the support of the ink sheet from the image receiving sheet so that the color image (magenta image) of the heat sensitive ink material is retained on the image receiving sheet; PA1 superposing a third heat sensitive ink sheet (such as a yellow ink sheet) on the image receiving sheet having the cyan image and magenta image thereon; PA1 placing imagewise a thermal head on the support of the second heat sensitive ink sheet to form and transfer a color image (yellow image) of the heat sensitive ink material onto the image receiving sheet; PA1 separating the support of the ink sheet from the image receiving sheet so that the color image (yellow image) of the heat sensitive ink material is retained on the image receiving sheet, whereby a multicolor image is formed on the second image receiving layer; and PA1 transferring thus prepared multicolor image together with the second image receiving layer onto a white paper sheet.
The fused ink transfer recording comprises the steps of superposing on an image receiving sheet a transfer sheet having support and a thermal fusible transfer layer which comprises a coloring material (e.g., pigment or dye) and imagewise heating the support of the transfer sheet to portionwise fuse the transfer layer to form an image on the image receiving sheet. A multicolor image also can be prepared using a number of color transfer sheets.
The fused ink transfer recording is advantageous in the sensitivity, cost, and endurance of the formed image, as compared with the sublimation dye transfer recording method. It, however, has the following drawbacks:
The color image prepared by the fused ink transfer recording is poor in its quality, as compared with the sublimation dye transfer recording. This is because the fused ink transfer recording utilizes not gradation recording but binary (i.e., two valued) recording. Therefore, there have been reported a number of improvements on the fusible ink layer of the fused ink transfer recording for modifying the binary recording to give a gradation recording so that a color image having multi-gradation is prepared by the fused ink transfer recording. The basic concept of the heretofore reported improvement resides in portionwise (or locally) controlling the amount of the ink to be transferred onto the image receiving sheet. In more detail, the mechanism of transfer of the ink in the fused ink transfer recording is as follows; under heating by means of the thermal head, the viscosity of the ink layer at the site in contact with the thermal head lowers and the ink layer tends to adhere to the image receiving sheet, whereby the transfer of the ink takes place. Therefore, the amount of the transferred ink can be controlled by varying elevation of temperature on the thermal head so that the cohesive failure in the ink layer is controlled and the gamma characteristic of the transferred image is varied. Thus, the optical density of the transferred ink image is portionwise varied, and accordingly, an ink image having gradation is formed. However, the optical density of a fine line produced by the modified fused ink transfer recording is inferior to that produced by the sublimation dye transfer recording method. Moreover, the optical density of a fine line produced by the modified fused ink transfer recording method is not satisfactory.
Further, the fused ink transfer recording has other disadvantageous features such as low resolution and poor fixation of the transferred ink image. This is because the ink layer generally uses crystalline wax having a low melting point as the binder, and the wax tends to spread on the receiving sheet in the course of transferring under heating. Furthermore, the crystalline wax scarcely gives a transparent image due to light scattering on the crystalline phase. The difficulty in giving a transparent image causes serious problems in the preparation of a multicolor image which is formed by superposing a yellow image, a magenta image, and a cyan image. The requirement to the transparency of the formed image restricts the amount of a pigment to be incorporated into the ink layer. For instance, Japanese Patent Publication No. 63(1988)-65029 describes that the pigment (i.e., coloring material) should be incorporated in the ink layer in an amount of not more than 20 weight % based on the total amount of the ink layer. If an excessive amount of the pigment is employed, the transparency of the transferred ink image is made dissatisfactory. Further, the Publication also disclosed continuous gradation recording by binary or multi-valued recording.
Improvements of reproduction of a multicolor image in the fused ink transfer recording method have been studied and proposed, so far. However, the study of the inventors have clarified that recording by the continuous gradation using the proposed heat sensitive recording material does not give an image having satisfactory continuity and stability of density. Further, the binary or multi-valued recording (i.e., image recording method utilizing multi-dots having different area one another; VDS (Variable Dot System)) using the heat sensitive recording material does not give an image having satisfactory continuity of density, transparency (especially transparency of multicolor image) and sharpness in edge portion.
In contrast, it is known that a thermal transfer recording can prepare a multicolor image having multi-gradation by means of the multi-valued recording which utilizes area gradation. Further, it is also known that a heat sensitive ink sheet which can be used in the multi-valued recording utilizing area gradation, preferably have the following characteristics:
In contrast, various image receiving materials (sheets) where an image formed in the heat sensitive ink sheet is transferred, are also proposed. For example, when the fused ink transfer recording is conducted using a plain paper sheet as the image receiving material, the resultant image shows defects such as unevenness in transferred area and lack of dots due to poor smoothness or ink-receiving property of a surface of the paper sheet. As image receiving materials almost free from these defects, there are known a synthetic paper, a synthetic resin film and a whit pigment containing paper.
In color proof where an image of heat sensitive ink material transferred on the image receiving sheet is further retransferred on a white paper sheet used for printing, it is required that an image transferred onto a white paper sheet is extremely near to a printed image in tone and surface gloss. However, use of the above image receiving material cannot give such an image.
As an image receiving sheet suitable for preparing the color proof, an image receiving sheet having two image receiving layers on a support sheet is disclosed in Japanese Patent Provisional Publication No. 2(1990)-244147). A first image receiving layer on the support sheet comprises polyamide or butyral resin, and a second image receiving layer provided on the first image receiving layer comprises a polymer such as poly vinyl chloride or vinyl chloride/vinyl acetate copolymer having softening point of not higher than 150.degree. C. and degree of polymerization of 200 to 2,000. The first image receiving layer functions in such a manner that an image of a heat sensitive ink material is easily and precisely transferred onto the image receiving sheet, and the layer is left on the support sheet after retransferring of the image. The second image receiving layer is transferred onto a white paper sheet together with an image of a heat sensitive ink material.
However, the resultant image using the image receiving sheet is not sufficiently satisfactory in formation of dot having appropriate size and shape and good reproduction of gradation, and the image is further is not sufficiently near to a printed image. Further, a surface of the image occasionally shows tackiness or sticking (sticking on other material such as sheet).
As for the thermal head printer, the technology has been very rapidly developed. Recently, the thermal head is improved to give a color image with an increased resolution and multi-gradation which is produced by area gradation. The area gradation means gradation produced not by variation of optical density in the ink area but by size of ink spots or lines per unit area. Such technology is described in Japanese Patent Provisional Publications No. 4(1992)-19163 and No. 5(1993)-155057 (for divided sub-scanning system) and the preprint of Annual Meeting of Society of Electrography (Jul. 6, 1992) (for heat concentrated system).
As a image forming method by transfer using the heat sensitive ink sheet, recently a method by means of laser beam (i.e., digital image forming method) has been developed. The method comprises the steps of: superposing the heat sensitive ink layer of the heat sensitive ink sheet on an image receiving sheet, and irradiating a laser beam modulated by digital signal on the heat sensitive ink layer through the support of the heat sensitive ink sheet to form and transfer an image of the heat sensitive ink layer on the image receiving sheet (the image can be further retransferred onto other sheet). In the method, the heat sensitive ink sheet generally has a light-heat conversion layer provided between the ink layer and the support to efficiently convert light energy of laser beam into heat energy. The light-heat conversion layer is a thin layer made of carbon black or metal. Further, a method for locally peeling the ink layer to transfer the peeled ink layer onto the image receiving sheet (i.e., ablation method), which does not fuse the layer in the transferring procedure, is utilized in order to enhance image quality such as evenness of reflection density of the image or sharpness in edge of the image.